realloc doesn't work unless memset is called

sydney-troz · 02-28-2008, 07:26 PM

Hi

I am somewhat new to C, although I have done a bit of programming in C++. My problem is this: I am attempting to double the size of a char * array using realloc() wrapped in the xrealloc() suggested in Libc's info page. However, after calling realloc() with all the right parameters (I've stepped through it with gdb), the block hasn't changed size -- that is, if the initial size was 2 elements, trying to access the 4th element segfaults. I'm still not sure why this happened, but I fixed it by memsetting the new half of the block (elements 2 + 3 in the previous example) to 0. After doing that in the xrealloc() wrapper, I can access the new memory just fine.

I Googled and searched various forums, and haven't been able to find an explanation for this, although I'm sure it's a basic element of the language that I missed somewhere in my reading. If anyone can take a few lines to enlighten me, that would be great.

Thanks

dmail · 02-28-2008, 07:38 PM

Was the original array created on the heap?
Can you show some code to demonstrate this please.

ta0kira · 02-28-2008, 08:07 PM

You're using the return value of xrealloc and not the parameter you passed to it, correct?
ta0kira

sydney-troz · 02-29-2008, 01:01 PM

xmalloc():

Code:

void *
xmalloc (size_t size) {
    register void *value = malloc (size);
    if (value == 0)
        abort(); /*fatal ("virtual memory exhausted");*/
    return value;
}

xrealloc():

Code:

void *
xrealloc (void *ptr, size_t size) {
    register void *value = realloc(ptr, size);
    if (value == 0)
        abort();
    return value;
}

Code that creates the array:

Code:

retn = (char **)xmalloc(sizeof(char *) * 2);

Code that resizes array:

Code:

retn = (char **)xrealloc(retn, num_mutations * sizeof(char *), 2 * num_mutations * sizeof(char *));

where num_mutations is the size of retn (initially 2).

retn is modified in a recursive function, it is initially allocated in the code for the base case, and resized by the recursive case. If you need more info, just ask.

ta0kira · 02-29-2008, 03:47 PM

In what context do you call memset? Are you saying that the original part of the array (the part that existed before xrealloc) is garbage unless you use memset? The added portion will always be garbage. Please post a brief main function that reproduces the problem (on your end, anyway.) Also, why does your call to xrealloc have 3 arguments?
ta0kira

sydney-troz · 02-29-2008, 08:28 PM

Hmm, that's weird: I was going through my code again to respond to some of your questions, and it worked. Seeing it with fresh eyes, I suspect that the problem I may have been having was that I was calling xrealloc on the new elements of retn without setting them to NULL or a valid pointer first, so it was trying to increase the size of a block that it didn't have access to. Thanks for asking questions, it seems to have brought some insight!